The Effect of Mainshock-Aftershock Seismic Sequences on the Occurrence Order and Position of Plastic Hinges

Previous earthquakes have shown that a strong ground motion is followed by some aftershocks that are smaller than the main shock, but often produce moderate to high aftershocks in the affected areas. Hence, the structures constructed in seismic areas are not only affected by a single seismic event, but this event also includes foreshocks, main shock and aftershocks. For example, the 2012 East Azerbaijan earthquake (August 21, 2012), with a magnitude of Mw = 6.4 occurred in the northeast of Tabriz, had an aftershock with a magnitude of Mw = 6.3 that happened approximately 11 minutes later. It is known that aftershocks can cause significant failure to the structures damaged by mainshock ground motions. In other words, during aftershocks, there are structures that have already been damaged by an earthquake and have not yet been repaired, which may be damaged or collapsed under the aftershock seismic event. Literature review shows that most existing codes are limited to choose a single event called "design earthquake", while the effects of aftershock earthquakes have been ignored. Despite the qualitative knowing of this issue, limited studies have been reported in the past studies on sequence earthquakes. The plastic hinge area in reinforced concrete buildings is an area where an RC member experiences a moderate to severe plastic deformation under the moderate to strong ground motions. The occurrence order and position of plastic hinges plays a key role in the seismic rehabilitation of old buildings as well as the design of new structures. Contrary to the subject importance, most studies have been limited to the steel structures, and no studies have been conducted on the occurrence order and position of plastic hinges in the reinforced concrete buildings under the mainshock-aftershock seismic sequences. Therefore, in this paper, three-dimensional models with 4, 7, 10, 13, 16, and 20 stories are evaluated under the seven single and seven mainshock-aftershock earthquake records by nonlinear time-history analysis. The formation and average rotation of plastic hinges as well as the performance level of the structures are calculated and compared by nonlinear time history analysis. The results show that the buildings suffered serious damage under the aftershock earthquakes. Number of plastic hinges that pass through LS level increase significantly, so that this number is 31 times in the 13-story building and the building collapse after mainshock earthquake. In all the structures except the 4-story building, under the mainshock-aftershock earthquake records, plastic hinges are formed in the columns of some stories.